Correction Appended

In office for just more than a month, West Virginia Sen. Joe Manchin skipped two decisive votes in the Senate’s lame-duck session Saturday — an immigration bill and the repeal of the military’s “don’t ask, don’t tell” policy — that could affect his future election prospects.

The repeal passed 65-31, while the DREAM Act granting a pathway to citizenship for illegal-immigrant children stalled. Manchin’s office said the Senator had been clear that he did not support either measure, but a prior engagement kept him from attending the weekend votes. Had he voted, Manchin would have been the lone Democrat opposing DADT repeal.

Manchin had “planned a holiday gathering over a year ago with all their children and grandchildren as they will not all be together on Christmas Day,” a spokeswoman said Saturday. “While he regrets missing the votes, it was a family obligation that he just could not break.”

However, skipping the votes brought even more attention to the Senator who, after winning a special election last month, is running again for a full term in 2012. Roll Call Politics rates his race a Tossup.

“I’m sure that most Senators, as well as the hundreds of staffers who had to come to work today, would have rather been at a Christmas Party like Joe Manchin,” National Republican Senatorial Committee spokesman Brian Walsh said in a statement going after one of the GOP’s top 2012 targets. “But perhaps in Joe Manchin’s world today was a win-win — not only was he able to skip work and party, but he was also able to avoid voting on two very sensitive political issues. For a Senator who has only been on the job a few weeks, Manchin’s absence today, and the apparent lack of seriousness with which he takes the job he was elected to do, speaks volumes.”

Despite his popularity as governor, Manchin could be among the most vulnerable Democrats in 2012 if Republicans can find a top-tier candidate to challenge him.

Another Democratic Senator running for re-election in a challenging state is Montana’s Jon Tester, who voted against the DREAM Act. His vote immediately elicited reactions online from the party’s liberal activist community, including a tweet from Daily Kos founder Markos Moulitsas that read, “Good luck getting reelected, [expletive].”

Moulitsas’ support in 2006 inspired the net roots to open their checkbooks and helped Tester win his long-shot election bid.

On the Republican side, Sen. Dick Lugar (Ind.) drew ire from conservatives for voting for the DREAM Act along with just a handful of his GOP colleagues. Conservatives and tea party activists said he is liberal for voting for what they consider to be an amnesty program. Lugar could face a primary challenge from the right in 2012.

Lugar voted against the DADT repeal.

Correction: Dec. 19, 2010

The article misstated the vote tabulation for the repeal of “don’t ask, don’t tell.”