But Van Hollen said a combination of poor strategic decisions by Republicans and a good campaign by Critz helped Democrats keep the seat on their side.

"It's very clear that Republicans did a test run of their November strategy in Pennsylvania 12 and it crashed, it failed," he said, adding that the GOP tried to "create bogeymen out of President Obama and Speaker Pelosi."

The congressman also emphasized the fact that Republican spending in the race outstripped the Democrats' $1.5 million to $1.3 million, arguing the GOP "went all in."

Following Critz's victory, Republicans said they would learn from the campaign and adjust their strategy. The Democratic candidate — who will be sworn into office Thursday — said he was opposed to the Democrats' healthcare bill and spoke mostly about jobs and the economy in a district where jobless rates are higher than the national average.

"Tonight’s result was undoubtedly disappointing, but we will take the lessons learned from this campaign and move forward in preparation for November," National Republican Congressional Committee Chairman Pete Sessions (Texas) said in a statement Tuesday. "They will steer clear of publicly campaigning with President Obama and Speaker Pelosi, distance themselves from the Democratic agenda, and attempt to co-opt Republican positions on the issues." 

Despite Republican skepticism, Van Hollen said that Critz's strategy is a winning one. Critz and Burns will likely face one another again in the fall.

"He was running as an independent-minded Democrat," Van Hollen said. "If you look at the races we won in 2006 and 2008, and others, we have members of Congress who vote their district. Their top priority, as it should be, are the people in their district and that's what Mark Critz ran on."

Even though it did not rear its head in the Pennsylvania race, Van Hollen said the Tea Party movement could cause problems for Republicans in the fall, citing Rand PaulRandal (Rand) Howard PaulNSA spying program overcomes key Senate hurdle Fix what we’ve got and make Medicare right this year Despite amnesty, DACA bill favors American wage-earners MORE's victory in the Kentucky Senate primary over GOP establishment candidate Trey Grayson.

The Maryland Democrat said "there's no doubt" there is more energy in the GOP, thanks to the Tea Party, but countered "that's begun to change.

"I think you're going to see that energy level change [in favor of the Democrats]," he said.