It's called playing the percentages. And numbers don't lie.

Since 1950 Democrats have won 13 of 21 U.S. Senate elections in Illinois, for a 62 percent victory quotient. Since 1980 Democrats have won nine of 10 Senate contests, or 90 percent.

However, since 1962, when Democrats fielded a white Chicagoan for senator (1962, 1966, 1968, 1972 and 1978), the suburban or Downstate Republican candidate won every time. When Democrats fielded a black Chicagoan for senator (1992, 1998 and 2004), the suburban Republican candidate won only in 1998. And, since 1980, when the Democratic Senate candidate was a Downstater, they triumphed in all seven races (1980, 1984, 1986, 1990, 1996, 2002 and 2008).

Hence, any competent odds maker would conclude that if the Democrats don't nominate a white Downstater or a black Chicagoan for U.S. senator in 2010, they likely will lose the seat occupied by Roland Burris, who was appointed to replace Barack Obama.

Burris is wisely retiring after he finishes his term, besmirched by improprieties surrounding his appointment by former governor Rod Blagojevich. The 2010 Democratic field includes state Treasurer Alexi Giannoulias, businessman Chris Kennedy, both white and both able to self-fund their campaigns, and Chicago Urban League president Cheryle Jackson, who is black. Giannoulias and Jackson are Chicagoans, and Kennedy, the son of the late Robert Kennedy, is a North Shore suburbanite.

The Republican field has been cleared for U.S. Representative Mark Kirk (R-10), also of the North Shore. An ideological moderate, with a pro-environment, pro-abortion rights, pro-gay rights, and pro-gun control voting record, Kirk is precisely the kind of Republican who loses party primaries but who is well positioned to win a statewide election -- provided, of course, that he's opposed by a white Chicago Democrat, not a Downstater.

And expect that to occur: The November 2010 matchup will be Kirk versus Giannoulias.

Here's a retrospective on past U.S. Senate contests in Illinois:

1962: Two-term incumbent Everett Dirksen, a Downstater from Pekin and the Republican minority leader, faced Chicago U.S. Representative Sid Yates. Dirksen won by 1,961,202-1,748,007, a margin of 213,195 votes. Yates won Chicago by about 300,000 votes, but he lost the suburbs and Downstate by more than 500,000 votes.

1966: Three-term Democratic incumbent Paul Douglas, a Chicagoan and a former Hyde Park alderman, faced suburban businessman Chuck Percy, who had lost the 1964 governor's race. In an anti-Democratic year, Percy prevailed by 2,100,449-1,678,147, a margin of 422,302 votes. Douglas won Chicago by fewer than 200,000 votes, and he was crushed in the suburbs and Downstate.

1970: After Dirksen's death in 1969, Republican Illinois House Speaker Ralph Tyler Smith of Downstate Alton was appointed to the vacancy. He faced state Treasurer Adlai Stevenson III, who resided in Lake County. Stevenson won by 2,065,054-1,519,718, a margin of 545,336 votes. He won Cook County by more than 600,000 votes, and Smith barely carried Downstate.

1972: Percy's alleged "liberalism" and anti-Nixon votes on key issues infuriated conservatives, but he was enormously popular as he positioned himself for a 1976 presidential bid. Democrat Roman Pucinski, a Northwest Side Chicago congressman, contrasted himself as a "workhorse" to Percy's "showhorse," but Pucinski had minimal appeal to blacks, suburbanites and Downstaters. Percy obliterated him by 2,867,078-1,721,031, a gargantuan margin of 1,146,047 votes. Percy got almost half the Chicago vote, running well in black wards, 69 percent of the vote in the suburbs (winning Cook County by 341,000 votes), and 65 percent Downstate (winning by 433,000 votes).

1978: Percy "fatigue" was rampant. Conservatives detested him. Illinoisans increasingly viewed him as an unprincipled opportunist. Chicago attorney Alex Seith tapped into that discontent and almost pulled an upset. Percy aired last-minute television ads apologizing for some of his prior votes and conduct. He won by 1,698,711-1,448,187, a margin of 250,524 votes. His Chicago vote dropped to 41.4 percent, and his Downstate vote fell to 55.9 percent. Only his 66.6 percent suburban margin saved him.

1980: For the first time since 1950, Democrats nominated a Downstater - Illinois Secretary of State Alan Dixon -- for senator. Dixon beat Seith in the primary by 671,746-190,339, and he faced Lieutenant Governor Dave O'Neal, a former Downstate sheriff, in the election. The result wasn't even close. Dixon won by 2,565,302-1,946,296, a margin of 619,006 votes. He got 78 percent of the Chicago vote, 53 percent of the suburban vote and 51 percent of the Downstate vote. In the same election, Ronald Reagan beat Jimmy Carter in Illinois by 376,636 votes. Dixon ran almost 585,000 votes ahead of Carter. The clear message: Downstate Democrats win.

1984: Percy, then the Foreign Relations Committee chairman, was in the political ozone, totally detached from Illinois. He was not a Reagan zealot, and he faced primary opposition from Tom Corcoran, a conservative Downstate congressman. Percy won by 387,865-239,847, a margin of only 148,018 votes -- a clear warning signal. Paul Simon, a 10-year Downstate congressman who lost the 1968 governor's race, saw an opportunity. He won the Democratic primary with just 35.6 percent of the vote, beating Chicago-based Phil Rock, Burris and Seith.

In the election, Simon replicated the Dixon model. He defeated Percy by 2,397,165-2,308,039, a margin of 89,126 votes. Simon won Chicago with 68 percent of the vote, held Percy to 56 percent in the suburbs and got 48 percent Downstate. In that election Reagan won Illinois by 620,604 votes, but Percy ran almost 400,000 votes behind Reagan while Simon ran 222,000 votes ahead of Walter Mondale. Obviously, a lot of Republicans refused to back Percy.

1992: That was the "Year of the Woman," and Chicagoan Carol Moseley Braun, the Cook County recorder of deeds, scored a huge Democratic primary upset. She got 557,694 votes (38.3 percent of the total), beating Dixon (504,077 votes) and Al Hofeld (394,497 votes). In the election, suburban Republican Rich Williamson stood no chance. The novelty of a black female candidate was too much to overcome, and Braun trounced Williamson by 2,631,229-2,126,833, a margin of 504,396 votes. Braun got 76 percent of the Chicago vote, 43 percent of the suburban and Collar County vote, and 53 percent of the Downstate vote. In that race, gender and party eclipsed race and geography.

1996: Simon retired after two terms, and the Democratic contenders were Dick Durbin, an obscure congressman from the Springfield area, and former state treasurer Pat Quinn, of Cook County. With Simon's backing, and with support from Chicago and Downstate party officials who reviled Quinn, Durbin won the primary 512,520-233,138, with 65 percent of the vote. The Republican frontrunner, Lieutenant Governor Bob Kustra, was upset in the primary by conservative Al Salvi, a McHenry-area state representative, 377,141-342,935, a margin of 34,206 votes. Had Kustra been nominated, he would have been elected, but Durbin, who had switched his position on abortion, effectively attacked Salvi as an "extremist."

Durbin crushed Salvi by 2,384,028-1,728,824, a margin of 655,204 votes. Durbin won Chicago with 80 percent of the vote, suburban Cook County with 56 percent and Downstate with 50 percent; he got 46 percent of the vote in the Collar Counties. A Downstate Democrat wins again.

 1998: Unlike Percy, who didn't suffer voter fatigue until his third term, fatigue with Braun manifested itself in her first term. After 6 years in office and some ethics transgressions, Braun was toxic. Republican Peter Fitzgerald, a conservative state senator from Palatine, beat state Comptroller Loleta Didrickson, a moderate, in the primary by 372,916-346,606, a margin of 26,310 votes.

Braun lost to Fitzgerald by 1,610,496-1,709,041, a margin of 98,545 votes. Fitzgerald carried 97 of the counties 102 counties, and he won Downstate by 322,000 votes, the Collar Counties by 170,000 votes and the Cook County suburbs by 14,000 votes; Braun won her Chicago base by 384,000 votes, getting 79 percent of the vote.

So, for Republicans, the formula for a 2010 Senate victory is clear: Lose Chicago by fewer than 400,000 votes, break even in suburban Cook County, win the Collar Counties by at least 125,000 votes, and win Downstate by 300,000 votes.

In a three-way primary, Jackson will get a sizable racial vote, but Giannoulias will edge her because of his Downstate support. Kirk faces a primary against Internet blogger Andy Martin, who will get up close and personal, using Kirk's divorce as an issue.

My prediction: Downstate Democrats win, Chicago Democrats lose. That's history. Kirk will surely be another Percy -- a RINO (Republican In Name Only). But if he can, like Percy in 1966, exploit discontent with the national Democrats, run well in the Cook County suburbs, and pound Chicagoan Giannoulias elsewhere, especially Downstate, he can win.

Unless Kirk implodes, as did 2004 nominee Jack Ryan after his divorce records were unsealed, he will be Illinois' next senator. He has historical "percentages" on his side. But, like Percy, his Washington record will alienate Republican conservatives, and, like Braun, his 2010 election will be an aberration, and he'll serve only a single term.