So what do you do if you're John Jacob, a Republican running against a five-term incumbent from your own party, Chris Cannon, and you learn the Republican White House has endorsed Cannon?

On Monday, a day after learning first lady Laura Bush is endorsing Cannon in a recorded phone message to voters in Utah's 3rd Congressional District, Jacob decided to criticize President Bush, saying the president supports illegal immigration and painting Cannon as Bush's water boy.

"Chris can't have it both ways," Jacob told KSL-TV. "He can be on the president's side, or he can be on the side that's against illegal immigration."

In an early morning debate on KCPW 1010 AM, Jacob said to Cannon, "You've been on the side with the president. You've rubber-stamped his ideas, he's coming out to support you, so if you win, you'll stay on that side."

Jacob said that as soon as Bush proposed a guest-worker program in January 2004, U.S. borders were inundated by people immigrating illegally in hopes of taking advantage of the plan. This year, Bush and Cannon have backed off on parts of the plan, and Cannon is now accusing Jacob of backing a type of amnesty.

The tougher talk about Bush is a calculated gamble by Jacob, who has been painted into a corner by Laura Bush's endorsement, said an elections analyst at Brigham Young University.

"What Mr. Jacob needs to do is make this calculation about who his supporters are, and that's a very active and vocal contingent on this issue who have criticized the president and who have shown in previous election cycles they're willing to put resources behind rhetoric," said Kelly Patterson, director of BYU's Center for the Study of Elections and Democracy.

"For a challenger in a primary to have really any chance of unseating an incumbent, he has to rely on this kind of outside help and influence and energy among a core of supporters."

One group supporting Jacob dialed down its rhetoric slightly over the weekend. The Team America political action committee removed from its Web site a picture of Cannon with a target over his face. But the site maintained a section on its front page dedicated to its effort to defeat "Chris 'King of Amnesty' Cannon in Utah."

Team America's ads deriding Cannon and urging Utahns to vote for Jacob also continued to run Monday on KSL 1160 AM.

"You talk about Chris having a target on him," Jacob told KSL radio talk-show host Doug Wright Monday, "but what kind of target do I have now" that Laura Bush has endorsed Cannon?

Patterson said it's no surprise a single issue is the major focus of the race.

"In a race where the two candidates are from the same party, their party doesn't differentiate them, even their ideology doesn't differentiate them. So they have to zero in on specific issue differences. The biggest is immigration."