Washington (CNN) It may be a no-brainer for Senate Republicans to keep President Donald Trump in office -- but it's becoming clear that Democrats mean to make them pay a heavy price for saving the President in his impeachment trial early next year.

In a sign of the high-stakes politics to come, minority Democrats are not waiting for the House to impeach Trump -- a move expected on Wednesday -- before they open the political battle in the Senate.

In an ironic touch, they are echoing the process complaints that House Republicans used to complicate Speaker Nancy Pelosi's strategy. And they complain that GOP plans for a swift Senate trial would subvert justice and fall short of the constitutional duty expected of senators.

In the House, Democrats used their majority power to force through a swift and focused impeachment strategy but opened themselves up to Republican charges that they were rushing the most grave undertaking faced by Congress. Now the situation is reversed.

Democrats lack the power to send Trump packing -- they would need 20 GOP senators to turn against their own party leader to amass a two-thirds majority to make him the first President ever ousted by Congress.

But the more Democrats can make a case that Trump's unchecked power grabs are being enabled by congressional Republicans, the better they can build a case to voters that it would be dangerous to reelect him next year.

"Conducting an impeachment trial in the Senate is an enormously weighty and solemn responsibility," Sen. Chuck Schumer, D-New York, said on Monday. The Senate minority leader's brief grin as he stepped up to his podium might have been a sign of relish at the chance to make Republican Majority Leader Mitch McConnell's life difficult.

The wily Senate veterans are expected to meet in the coming days to begin to discuss terms for the trial, an encounter that will test their instinctive feel for their respective caucuses and flair at manipulating arcane Senate procedure for their own benefit.

Already, there is a handful of Republican senators , including moderates up for reelection, seasoned veterans who are retiring, and outright critics of Trump, whose views on the trial are a concern to the White House and GOP leaders. They're worried some of that group could break and vote with Democrats on key trial-related issues, sources tell CNN.

The House is expected to vote on Wednesday on two articles of impeachment . The first alleges that Trump abused power by withholding nearly $400 million in military aid to Ukraine in a bid to coerce its President into investigating Democrats, including his possible 2020 opponent Joe Biden and his son Hunter.

The second claims that Trump obstructed Congress by refusing to turn over documents and to allow insiders such as former national security adviser John Bolton and White House acting chief of staff Mick Mulvaney to testify in the House impeachment probe.

McConnell told reporters he would have more to say about Schumer's proposals, delivered in an open letter, on Tuesday. But his political task became even more delicate following the latest extraordinary intervention by the President's personal lawyer Rudy Giuliani that appeared to prove an aspect of the Democratic case.

Giuliani said in an interview with The New Yorker that he needed former US Ambassador to Kiev Marie Yovanovitch "out of the way" for the back-door diplomatic mission he conducted on behalf of Trump to succeed.

His remark was the latest on-the-record comment, including those by Trump and Mulvaney, that bolster the case that the President used his power to advance personal political goals in Ukraine. The mounting evidence is one reason why GOP House members resorted to spreading misinformation to fog the public's understanding of the case.

Inflicting pain

There's not much to be said for being in the minority -- especially during an impeachment drama. But as House Republicans will likely show Tuesday in a House Rules Committee meeting to set parameters for a full floor debate and vote, a party out of power can stir up trouble for those with the gavel.

As the political dynamic in Congress shifts, Schumer is moving quickly before the holiday season, seeking to tilt the battlefield ahead of a Senate trial next month.

Schumer appears to be trying to achieve multiple objectives.

First, he can maximize the political heat on Republicans and try to ensure that as many Americans as possible come away from a Senate trial reasoning that the President's party shielded him from the consequences of serious misconduct.

His call for witnesses likely to damage Trump's case may also be a warning shot -- to warn McConnell how Democrats would respond if the President tries to turn the proceedings into a circus that would delight his conservative media friends.

Schumer may also be targeting swing-state Republicans in the short-term who in the event that he cannot reach agreement on trial arrangements with McConnell will help shape the process in a series of majority votes.

His call for witnesses also undercuts a Republican defense that the Democratic case against Trump is incomplete and based on hearsay -- since he is asking for key witnesses.

"Senate Democrats believe strongly that the trial must be fair and it's very important that the American people judge it to be fair," he said, only hinting at the political implications for Republicans if he can prove that not to be the case.

But Republicans might note that House Democrats deigned to pursue witnesses such as Bolton and Mulvaney to the full extent of the law in court.

The gambit comes as new polls show that just under half of Americans believe that Trump should be impeached and removed from office. A CNN poll released Monday found that 45% of Americans support impeaching and removing Trump, down from 50% last month. A recent Fox News poll had 50% of respondents saying that Trump should be impeached and removed from office.

Grassroots Republican voters form a firewall for Trump, meaning that there will never be a two-thirds Senate majority to throw him out of office. But so far, the focus has been on House Democrats who are exposed because they face reelection in districts won big by the President in 2016.

Vulnerable swing state Senate Republicans, especially those such as Colorado's Cory Gardner or Maine's Susan Collins, must also consider the political consequences of a vote to acquit Trump.

Unlike colleagues in deep red Trump states or House members in gerrymandered districts, senators from purple states face a delicate balance in their decision making, even if they eventually opt to side with Trump.

In their zeal to show loyalty to the President, Republicans may have given Schumer an opening.

McConnell told Fox News that he was working in lockstep with the White House on plans for the trial. And South Carolina Sen. Lindsey Graham told CNN International that he did not feel under pressure to pretend to be a "fair juror."

The comments allowed Democrats to portray Republicans as partisan and flouting their constitutional duty to hold a President to account, even if they will do nothing to alter the broader calculation that will keep Trump in office.

Republican jurors on the spot

Schumer's initiative is already making for some uncomfortable moments for Republicans -- a taste of the harsh spotlight they can expect when the trial gets under way.

Collins told reporters on Monday that it was unfortunate that Schumer had published his letter before sitting with the majority leader to work out arrangements for a trial.

But she also displayed discomfort with McConnell's statements about coordinating with the White House.

"Every senator has to decide on his or her own how to approach it. That would not be the approach that I've taken," Collins said. CNN's Manu Raju caught North Carolina Sen. Richard Burr as he stepped into an elevator and asked him whether he wanted to hear from trial witnesses.

"No," said Burr.

Utah Sen. Mitt Romney, one of the Republicans on whom Democrats are trying to work, said he had a point of view about the witnesses question, but wasn't ready to discuss it.

There was also an embarrassment for McConnell after footage emerged of the senator calling for witnesses 20 years ago on CNN's "Larry King Live" during the Clinton impeachment saga.

It is unclear how serious Schumer might be about bargaining with McConnell and if he would be willing to trade for any witnesses from whom Republicans might wish to hear.

Some Trump loyalists have been angling for an appearance from Biden's son, or even the former vice president himself.

Judging by eye-opening details unveiled by Hunter Biden about his former conduct and personal life in previous media interviews, he would be an unpredictable witness.

It is also unlikely that Biden's presidential campaign has any interest in either their candidate or his son taking a stand -- in appearances that would allow Trump to escalate his narrative that the two men did something wrong -- claims that are not supported by any evidence.