Sen. Mark UdallMark Emery UdallKennedy apologizes for calling Haaland a 'whack job' OVERNIGHT ENERGY: Haaland courts moderates during tense confirmation hearing | GOP's Westerman looks to take on Democrats on climate change | White House urges passage of House public lands package Udalls: Haaland criticism motivated 'by something other than her record' MORE (D-Colo.) and Rep. Cory GardnerCory GardnerBiden administration reverses Trump changes it says 'undermined' conservation program Gardner to lead new GOP super PAC ahead of midterms OVERNIGHT ENERGY: Court rules against fast-track of Trump EPA's 'secret science' rule | Bureau of Land Management exodus: Agency lost 87 percent of staff in Trump HQ relocation | GM commits to electric light duty fleet by 2035 MORE (R) both courted controversy in the second debate of their Senate battle on Monday afternoon.

The vulnerable Democrat stood by his vote for ObamaCare and expressed support for a carbon tax, though he dodged when asked specifically if he would support a cap-and-trade bill.

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Affirming a line of attack from Republicans, Udall also said he voted for $800 billion in cuts to Medicare Advantage, but he said the cuts went to "shoring up [Medicare's] solvency."

And Gardner tacked to the center on immigration, declaring that the "DREAM Act is going to be part of the solution on immigration reform" and that "an earned [legal] status for [illegal immigrants] is going to be part of the solution."

It was a more aggressive performance from Udall than voters saw in the candidates' first debate.

He called Gardner "missing in action" on the governor's task force for tackling healthcare reform, and charged that though Gardner says he's for immigration reform, "he hasn't lifted a finger" to help pass reform in the House.

He accused Gardner of supporting the government shutdown "out of misplaced loyalty to the Tea Party," a move he called "irresponsible" because the shutdown occurred while Colorado was reeling from heavy flooding.

"At our greatest time of need, an ideology took a hold of Congressman Gardner when we needed all hands on deck," Udall said.

Gardner, who voted for the GOP budget that defunded ObamaCare and was the trigger for the shutdown a year ago, said he hadn't supported the shutdown, and accused Udall of "politicizing a tragedy" with respect to the flooding.

The congressman worked to keep Udall on the defensive throughout the debate, peppering him with questions meant to nail him down on energy and healthcare.

He asked whether Udall "broke your word when you said you could keep your insurance," a reference to the thousands of Americans who lost their insurance coverage under ObamaCare. Udall noted that many who received cancellation notices were offered the opportunity to renew their coverage at the same time, but admitted there were issues with the law that needed fixing.

"We're gonna make the Affordable Care Act work. It's not perfect, you all know that, but we're going to make it work," he said.

But he later added: "Hindsight is 20-20. There are some changes I would've made" if he had the opportunity to redo the law.

And Gardner repeatedly asked Udall to name a price he'd attach to carbon emissions, at one point drawing murmurs from the crowd — all questions Udall dodged.

While Udall framed the race as one of contrasts, between an incumbent who wants to "lean forward" and "create our future," and Gardner, who is "looking backwards."

"Congressman Gardner talks about being a member of the next generation and a new Republican — well the next generation doesn't want to shut off science, and the next generation doesn't want to shut out immigrants, the next generation doesn't want to shut down government," Udall said in closing.

He added that "all generations see things my way."

Gardner, meanwhile, kept the focus on President Obama, repeatedly referencing the president's recent comments that his policies "are on the ballot this fall."

"On the ballot in just a few weeks will be the president's policies. Mark Udall has voted for the president's policies 99 percent of the time, and that's what we're going to be voting on," he said. "The president admitted that just the other day, and in the newspaper over the weekend the senator said that he votes with the president when he agrees with him. Which means, I guess, he disagrees with him only 1 percent of the time."

Obama remains deeply unpopular in Colorado, and that unpopularity is one of the main reasons Udall has become one of Democrats' most vulnerable incumbents this cycle, with most public polling showing him tied with Gardner.