President Barack Obama suggested strongly on Friday that Russia's Vladimir Putin knew about the email hackings that roiled the U.S. presidential race, and he urged his successor, Republican Donald Trump, to back a bipartisan investigation into the matter.

"Not much happens in Russia without Vladimir Putin," Obama said in his year-end news conference. The president said he had warned Putin there would be serious consequences it he did not "cut it out," though Obama did not specify the extent or timing of any U.S. retaliation for the hacking, which many Democrats believe contributed to Trump's victory over Hillary Clinton.

Obama also expressed bewilderment over Republican lawmakers and voters alike who now say they approve of Putin, declaring, "Ronald Reagan would roll over in his grave."

Trump has dismissed recent talk about hacking and the election as "ridiculous."

Clinton has even more directly cited Russian interference with the U.S. election. She said Thursday night, "Vladimir Putin himself directed the covert cyberattacks against our electoral system, against our democracy, apparently because he has a personal beef against me."

Obama did not publicly support that theory Friday. He did, however, chide the media for that he called an "obsession" with the flood of hacked Democratic emails that were made public during the election's final stretch.

U.S. intelligence assessments that Russia interfered in the election to benefit Trump have heightened the already tense relationship between Washington and Moscow. Separately, Obama has blamed Russia for standing in the way of international efforts to stop the civil war in Syria, where government forces have beaten back rebels in their stronghold of Aleppo.

Obama said he feels "responsible" for some of the suffering in Syria, but he defended his decision to avoid significant military action there. He said that while military options short of invasion were tempting, it was "impossible to do this on the cheap."

Still, he pinned the bulk of the blame on Russia, as well as Iran, for propping up Syrian President Bashar Assad.

"This blood and these atrocities are on their hands," he said as he addressed reporters from the White House briefing room shortly before leaving on his annual Christmas vacation to Hawaii. The news conference lasted about an hour and a half, longer than usual.

The president is ending his eighth year in office with his own popularity on the rise, though Trump's election is expected to unwind many of Obama's policies. He's leaving his successor a stronger economy than he inherited, but also the intractable conflict in Syria and troubling issue of whether Russia was meddling in the U.S. election to back Trump.

U.S. intelligence agencies have concluded with "high confidence" that Russia interfered in the election on Trump's behalf. The president-elect has disputed that conclusion, setting up a potential confrontation with lawmakers in both parties.

The president rejected any notion that the dispute over the origin of the hacking was disrupting efforts to smoothly transfer power to Trump. Despite fiercely criticizing each other during the election, Obama and Trump have spoken multiple times since the campaign ended.

"He has listened," Obama said of Trump. "I can't say he will end up implementing. But the conversations themselves have been cordial."

The president did weigh in on Trump's decision to speak with the leader of Taiwan, a phone call that broke decades of U.S. diplomatic protocol. Obama advised Trump to "think it through" before making changes the "one-China" policy, in which the U.S. recognizes Taiwan as part of China.

Trump has openly questioned why the U.S. upholds that policy, particularly given that Washington has other contacts with Taiwan. Offering his own take, Obama noted that Taiwan is of utmost importance to the Chinese and Beijing could have a significant response to any change in U.S. policy.

Trump's election has upended the Democratic Party, which expected to not only win the White House but also carry the Senate. Instead, the party finds itself out of power on both ends of Pennsylvania Avenue.

In a moment of self-reflection, Obama acknowledged that he had not been able to transfer his own popularity and electoral success to other sin his party.

"It is not something that I've been able to transfer to candidates in midterms or build a sustaining organization around," Obama said. "That's something I would have liked to have done more of but it's kind of hard to do when you're dealing with a whole bunch of issues here in the White House."

As he leaves office, the president has said the shaping the future of the Democratic Party now falls to others. But he all but endorsed his Labor Secretary Tom Perez to head the Democratic National Committee, lavishing praise on his Cabinet aide.

The DNC leadership elections have become a proxy fight in the broader battle for control of the party after eight years of Obama's leadership.