Challenging an incumbent Democratic governor in a key presidential swing state, Democrat Claire McCaskill (search) thinks she has hit upon a winning message: If Democrats dump Gov. Bob Holden (search), she can help them win the White House.

With Missouri's Aug. 3 party primaries drawing near, public opinion polls show McCaskill in a virtual dead heat with Holden, and Democratic presidential candidate John Kerry (search) in a similarly close race against Republican President Bush.

So McCaskill, the state auditor, is pressing the issue of how Democrats can best win in November. Put simply: "I can help Sen. Kerry better than Gov. Holden," McCaskill said.

McCaskill is running against the odds. An incumbent governor hasn't lost a primary in the United State since 1994.

Should she prevail, her odds of winning the general election rise significantly. Of the 17 challengers who knocked off same-party incumbents since 1970, 12 went on to win the governor's office, according to research by Thad Beyle, a political scientist at the University of North Carolina at Chapel Hill.

As of right now, "it looks pretty tough for the governor" of Missouri, Beyle said.

Holden is putting up a vigorous defense by comparing himself to President Truman, Missouri's Democratic icon who won a stunning 1948 election by campaigning against a "do-nothing" Republican Congress. During a recent debate, Holden proclaimed himself "the governor that stood the test of time" against Missouri's Republican-led Legislature.

But his term hasn't been smooth.

Republicans won control of the Senate for the first time in a half-century. The Legislature grew resistant. And much of Holden's campaign platform has been undermined by budget shortfalls.

Holden also achieved a rare distinction last year when lawmakers overrode three of his vetoes - matching in one year what had been the total number of vetoes overridden in Missouri since the Civil War.

The state's two largest papers, the Kansas City Star and the St. Louis Post-Dispatch, endorsed McCaskill in editorials published Sunday.

Kerry has endorsed no one in Missouri's gubernatorial primary and hasn't visited the state since June 3.

Kerry's campaign insists the Democratic infighting hasn't kept him away from Missouri; Kerry plans to come back as part of his 21-state swing after this week's Democratic National Convention. Yet, the uncertainty of the gubernatorial primary has made campaigning in Missouri problematic for Democrats.

"We're looking forward to moving past the primary and having a (gubernatorial) nominee and getting to a point where there is solidarity behind the ticket," said Kerry's Missouri campaign spokesman, Michael Golden.

Holden, 54, and McCaskill, 51, share similar political roots.

Both won election to the Missouri House in 1982 and served six years before leaving public office. After working for Rep. Dick Gephardt, Holden won two terms as state treasurer then narrowly captured the governorship in 2000. McCaskill served as a prosecutor before winning two terms as state auditor.

Now their energies are turned against one another. Holden accuses McCaskill of political opportunism for taking campaign donations from businesses upset over Holden's opposition to concealed guns and his effort to close tax loopholes. McCaskill criticizes Holden for his lavish, $1 million inaugural party, frequent early-term jet trips, education funding cuts and Missouri's potholed roads.

Holden has unsuccessfully called for tax increases on tobacco, casinos and wealthy Missourians to fund education and health care. McCaskill opposes tax increases, instead vowing to implement 100 of her moneysaving audit recommendations in her first 100 days as governor.

A poll conducted for The Kansas City Star and KMBC-TV and released last week, found Holden leading McCaskill 43 percent to 41 percent, with 16 percent undecided. The poll's margin of error was plus or minus 5.7 percentage points.

Another recent poll conducted for the Star and KMBC-TV showed McCaskill with a 45 percent to 37 percent advantage in a hypothetical November matchup against Republican gubernatorial front-runner, Secretary of State Matt Blunt. In the same poll, Holden garnered 42 percent support against Blunt's 44 percent. The poll had a margin of error of plus or minus 4 percentage points.

Seizing on that, McCaskill has enlisted former Gov. Roger Wilson to send a letter to fellow Democrats next week highlighting the national implications of the governor's race. In an interview, Wilson praised McCaskill's dynamic speaking ability - a contrast to Holden, whose speaking pace and tone rarely come off as forceful.

A Holden campaign spokesman said the best Democratic ticket would include the incumbent.

"I think Bob Holden's record of fighting for Democratic principles and the accomplishments he's had will complement (Kerry's) message better," said Holden spokesman Caleb Weaver. "There are a lot of similarities in the way these two men have approached their political life."