A rising star in the Republican Party has dimmed over the past week.

House Minority Whip Eric Cantor (Va.), a politically shrewd up-and-comer in the GOP, has broken with his party on two high-profile issues. And the defections on last week’s AIG bonus tax bill and the Obama administration’s troubled assets plan have exasperated some members in the GOP conference.

The grumbling started when Cantor unexpectedly voted with Democrats last week on a measure to recoup the bonuses of AIG executives. Many Republicans called the bill unconstitutional, with more than half of the GOP conference rejecting it. Cantor, who has been labeled “Mr. No” by some Democrats, was one of only two Republican leadership officials who voted for the bill. Rep. Cathy McMorris Rodgers (Wash.) was the other.

“All the unconstitutional stuff aside, if you don’t believe in raising taxes, why would you vote to raise taxes?” House Republican Conference Secretary John Carter (Texas) said.

Rep. Louie Gohmert (R-Texas) added that supporting the retroactive tax cut “sets a terrible precedent, just terrible.”

But Cantor voted for that retroactive tax, saying that he didn’t want to “reward failure,” meaning the AIG officials would keep their bonuses if the legislation hadn’t passed.

Yet Cantor’s vote last Thursday now looks even worse to some of his GOP colleagues. AIG employees are vowing to give their bonuses back and Democratic leaders say their bill — even if it’s not passed — accomplished its mission. The legislation appears to be dead.

The vote last week was a tough one for most Republicans amid the public’s outrage over the $165 million in bonuses after the company received $170 billion in bailout funds. Most politically vulnerable Republicans backed the Democratic bill, but there were some conservatives other than Cantor who voted yes, including Reps. Dave Camp (Mich.), Joe Barton (Texas), Paul Ryan (Wis.) and Roy Blunt (Mo.), who is running for the Senate.

A Republican legislator said, “Some members wanted to stick it to AIG — bonuses and recipients — but at some point in time your obligation is to stand in front of the mob and say, ‘Look, we’re going in the wrong direction, let’s think about this.’ ”

Another Republican suggested more members might have voted no if Cantor had.

The GOP legislator who rejected the Democratic bill said sarcastically, “When your whip votes against you, it’s kind of tough to whip for it.”

Cantor’s colleagues in leadership called the bill “a sham.” Minority Leader John Boehner (R-Ohio) said that it was “more than an attempt to cover someone’s rear end because of the political damage that’s out there.”

Boehner told reporters last Thursday that he did not know how Cantor was going to vote on the bill. Boehner voted no early on, while Cantor waited until late to register his vote of yes.

But a leadership aide privy to conversations among GOP leaders said that in the end, Cantor “got spooked.”

Throughout his career, Cantor has made many wise political moves as he climbed the leadership ladder. Some thought he could have supplanted Boehner as GOP leader this year, but Cantor opted to be patient and easily claimed the No. 2 GOP post in the House.

Cantor remains quite popular among his Republican peers, though his handling of the AIG bill has some members believing he has more to learn before he can helm the GOP conference.

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The 45-year-old lawmaker issued a statement that excoriated the administration’s proposal.

Meanwhile, Wall Street and Republican leaders in the Senate embraced the plan as the Dow Jones Industrial Average spiked nearly 7 percent.

Cantor called it a “shell game that hides the true cost of the program from the taxpayers that will be asked to pay for it.”

The top Republican on the Senate Budget Committee labeled it “a genuine and sincere effort.”

Boehner distanced himself from Cantor’s characterization of the assets initiative.

Boehner told reporters on Tuesday that Republicans were going to take a wait-and-see approach before offering an alternative.

“We’ll wait for more details before we prescribe what we think would be a better solution,” Boehner said.

Senate Minority Leader Mitch McConnell (R-Ky.) said on Monday that he was willing to “give the secretary of the Treasury credit for finally turning to the real issue here.”