WASHINGTON — After President Bush showcases his 2007 priorities in tomorrow’s State of the Union speech, one of the biggest obstacles to enacting his agenda will be his own party’s lawmakers, who lost control of both houses of Congress in November.

Disillusioned by an administration they blame for election losses, Republicans are displaying a heightened willingness to challenge Mr. Bush on everything from the Iraq war strategy to federal funding for stem-cell research. At least two dozen House Republicans crossed party lines to vote for each of the six items on the Democrats’ agenda in the first 100 hours of the new Congress this month.

The election losses are “a huge motivator for some to part with the president, even if they agree with him,” Rep. Chris Shays, a Republican of Connecticut, said.

Any suggestion by Mr. Bush to extend the tax cuts passed by Congress when it was controlled by Republicans will run into bipartisan opposition. Democrats are working to block the extensions by reinstating a rule requiring future tax cuts to be offset with spending cuts or fee increases to avoid raising the deficit. Some Republicans agree.

Senator Voinovich, a Republican of Ohio, said he is prepared to vote with Democrats against big tax cuts and for the so-called “pay-go” budget rule.

“We can’t afford to lose another dime, and by the way, we’re at war,” Mr. Voinovich said.

Mr. Bush’s speech will mark his seventh such address since he came to office in January 2001. White House spokesman Tony Snow said last week that it will focus on a handful of issues, including the Iraq war, energy independence, healthcare, immigration, and education. “He’s going to lay a way forward for Democrats and Republicans to work together,” Mr. Snow said.

On at least two of those issues, immigration and Iraq, Mr. Bush will face significant numbers of his own party steadfast in opposition.

While the Senate minority leader, Senator McConnell, a Republican of Kentucky, said he will work with Democrats on immigration, House Republican leaders still oppose Mr. Bush’s proposal to give illegal immigrants a chance at citizenship.

“I could not support a plan that allows citizenship as a reward for coming into the country illegally,” the House minority whip, Rep. Roy Blunt, a Republican of Missouri, told reporters last week.

The Iraq war has drawn the most visible opposition from Republicans to Mr. Bush. His proposal to send another 21,500 troops to Iraq to quell insurgencies has been met with opposition from at least a dozen Senate Republicans, and so far two have signed onto a non-binding resolution stating opposition to the policy.

The war has helped drag down Mr. Bush’s approval ratings. A Bloomberg News/Los Angeles Time poll conducted January 13 to 16 found just 39% of adults surveyed approved of the job he’s doing, while 59% disapproved. The poll had a margin of error of plus or minus 3 percentage points.

Republicans already paid a price in November and are looking to restore the health of the party, said Craig Shirley, an Alexandria, Va.-based Republican consultant.

Twenty-one Republican-held Senate seats will be on the ballot in 2008, compared with just 12 seats held by Democrats.

In the House, all 435 seats will be on the ballot. Of those, 41 are now held by Republicans who won in 2006 with 55% of the vote or less, a level that often attracts strong challengers.

“Anybody who is up for election in 2008 has reason to be concerned because the Republican Party is so heavily invested in the war against terror and the war in Iraq,” Mr. Shirley said.

Mr. Bush’s last two State of the Union addresses pushed some issues that splintered his party and didn’t become law even with Republicans in control of Congress.