Washington (CNN) President Barack Obama has vowed the US will retaliate against Russia "at a time and place of our own choosing" for Moscow's hacking attempts to influence the country's elections.

A growing chorus of powerful voices on Capitol Hill -- including Mitch McConnell, the top Republican in the Senate -- is calling for a bipartisan probe into Russian interference in the 2016 presidential vote.

But President-elect Donald Trump has rejected out of hand any suggestions of Russian influence on the election -- despite the CIA concluding that Russia acted to help Trump win.

So why is Trump so quick to dismiss intelligence agencies' findings even as leaders of his own party express concerns about Russia's role in the 2016 election? And how conclusive are the US intelligence community's findings?

Let's dig in.

What does the US believe Russia did to interfere in the 2016 campaign?

The US government publicly announced in October that it was "confident" Russia orchestrated the hacking of the Democratic National Committee and other political organizations of the Democratic Party.

Those hacks resulted in the public release of thousands of stolen emails, many of which included damaging revelations about the Democratic Party and former Secretary of State Hillary Clinton, the party's nominee.

But intelligence agencies didn't go as far as suggesting the efforts were aimed at bolstering Trump's chances and hurting Clinton's. Then, earlier this month, the CIA announced to a group of top US senators its latest finding: that Russia's hacks were aimed at helping Trump.

So what changed?

The CIA's new conclusion was based on its latest and most complete analysis of intelligence on the hacking, including the finding that Russian hackers breached GOP individuals and organizations prior to the election, including Republican House members, thought leaders and non-profits to the GOP, a former senior law enforcement official with direct knowledge of the hack investigation told CNN.

There is also evidence that entities connected to the Russian government were bankrolling "troll farms" that spread fake news about Clinton. Investigators also found digital footprints of individuals tied to the Russian government who had been on intelligence agencies radar before, as was acknowledged when the intelligence agency put out a public statement in October.

Republican National Committee officials have repeatedly denied that their systems were breached, insisting instead that only individual staffers' accounts were hacked.

The US intelligence community writ large is increasingly confident the Russian hacks were aimed at helping Trump, but the 17-agency intelligence community has not officially drawn that conclusion.

JUST WATCHED McCain concerned about Russia hacking Replay More Videos ... MUST WATCH McCain concerned about Russia hacking 09:21

What's Obama's take on all of this?

Obama says he is determined to take action against Russia -- and says Putin is "well aware of my feelings about this, because I spoke to him directly about it."

"I think there is no doubt that when any foreign government tries to impact the integrity of our elections that we need to take action and we will at a time and place of our own choosing," he told NPR.

But the outgoing US President didn't reveal what form the country's retaliation was likely to take: "some of it may be explicit and publicized; some of it may not be," he said.

A variety of options are on the table, according to one US official, but everything is being carefully considered because "right now it's like the old cold cyber war and the last thing you want to do is turn it into a hot shooting war."

Time is running out for Obama to act -- his presidential term ends in five weeks, on January 20 -- and Trump, his replacement in the White House, could opt to reverse any sanctions or other punishments imposed on Russia.

How is Trump reacting?

The President-elect and his transition team have been quick to rebuff the new intelligence assessment and dismiss out of hand any concerns about Russian influence in the election.

"These are the same people that said Saddam Hussein had weapons of mass destruction," Trump's transition team said in a statement hours after the Washington Post reported Friday on the CIA's latest assessment in a startling effort to discredit US intelligence officials.

And on Sunday, Trump called the assessment flat-out "ridiculous."

"I think it's just another excuse. I don't believe it. I don't know why and I think it's just -- you know, they talked about all sorts of things. Every week it's another excuse...No, I don't believe that at all," Trump said. "They have no idea if it's Russia or China or somebody. It could be somebody sitting in a bed some place. I mean they have no idea."

President Barack Obama has ordered US intelligence agencies to complete a full review of hacking in US elections going back to 2008 before he leaves office and Trump is sworn in to take his place.

What is Trump basing his dismissal of the US intelligence officials' conclusions on?

Trump hasn't offered any evidence to counter the CIA claims, other than to insist it is a political effort aimed at delegitimizing his electoral victory -- also without evidence.

His rebuttal is nearly identical to his pushback during the 2016 campaign after US officials began concluding Russia orchestrated the hacks of the Democratic Party's political organizations -- when he repeatedly said he did not believe US intelligence and law enforcement conclusions about Russia's responsibility for the hacking.

"I don't think anybody knows it was Russia that broke into the DNC. She's saying Russia, Russia, Russia," Trump said during the first presidential debate. "Maybe it was. I mean, it could be Russia, but it could also be China, but it could also be lots of other people, it also could be someone sitting on their bed that weighs 400 pounds, OK?"

JUST WATCHED CNN host, RNC strategist clash over CIA intel Replay More Videos ... MUST WATCH CNN host, RNC strategist clash over CIA intel 02:31

There is some dissent in the US government over the CIA report, right?

Yes, law enforcement officials have said that the FBI hasn't concluded the RNC was directly breached or that Russian hacking was done to help Trump win.

The FBI did find that a third-party group holding data belonging to the RNC was hacked and that conservative groups and pundits were hacked.

But many top Republicans are accepting the US intelligence community's assessments...

Yes, most congressional Republicans accepted intelligence and law enforcement officials' conclusions that Russia meddled in the presidential campaign and orchestrated the hack of Democratic groups.

And in light of the latest findings, both Republicans and Democrats have issued calls for a deeper probe of Russian interference in the 2016 election.

The calls began with a letter signed by the incoming Senate Democratic leader Sen. Chuck Schumer, Armed Services Committee chairman Sen. John McCain, Sen. Lindsey Graham and the Armed Services Committee's ranking member Sen. Jack Reed.

Senate Majority Leader Mitch McConnell issued his own call for a congressional investigation of the Russian hacking in a press conference Monday, in a notable break with Trump.

He expressed "the highest confidence in the intelligence community, and especially in the Central Intelligence Agency" and said "it defies belief that somehow Republicans in the Senate are reluctant to either review Russian tactics or ignore them."

Asked about McConnell's support for a deeper investigation into Russia's role in the election, Trump spokesman Jason Miller called the ongoing focus on the role of Russian hacking the election "an attempt to try to delegitimize President-elect Trump's win."

So what's next?

Members of Congress are likely to agree to launch some kind of investigation into the Russian hacking.

But Democrats are also ramping up their calls for a public accounting of the CIA's findings, with congressional Democrats calling for the Obama administration to declassify the CIA's report, at least in part.

And the Clinton campaign on Monday backed an effort by mostly Democratic Electoral College electors demanding a classified briefing on the Russian hacking before they vote next week to officially elect the next president of the United States.

Meanwhile, Russia's targeted hacking of US political organizations is continuing unabated, despite attempts to block access, according to US officials.

Russians "continue to do all kinds of stuff" against American political organizations, think tanks and thought leaders, one official told CNN. "It's not like the one and done deal here. They continue to engage in this operation around the clock."

CNN's Jim Sciutto and Pamela Brown contributed to this report.