Washington(CNN) The White House has been quietly pressuring Republicans in recent days not to buck President Donald Trump and vote in favor of a resolution that would overturn his national emergency declaration on the southern border, warning those who don't get in line that there could be 2020 election consequences.

The Senate is expected to vote midweek next week on the resolution of disapproval, which passed the Democratic-controlled House last month 245-182.

Four Republican senators -- Susan Collins, Lisa Murkowski, Rand Paul and Thom Tillis -- have already said they will support the resolution.

Trump has pledged to veto the measure and it's doubtful lawmakers will have enough support to override it. But White House officials are attempting to limit the number of GOP defections ahead of the vote next week by telling them the President is paying close attention to who votes against him.

Trump's staff is looking at this two ways: If only four Republicans support the resolution to overturn his emergency declaration, it won't be as significant of a problem because some opposition is expected. But if that number grows to, say, 10 Republican senators or more, officials are worried about the potential negative optics of opposition from many members of his own party.

White House aides want to avoid another negative news cycle -- one about Republicans breaking with the President -- and don't want lawmakers to see this as an inconsequential vote.

In recent days, White House officials have conveyed a stern message to GOP senators, especially those up for re-election in 2020: There will consequences if they vote with the Democrats and defy the President. They have vowed it will affect their standing with the administration and there will be retribution, including potential primary fights.

The President said as much in a Wednesday tweet, urging his party to stay united.

"Senate Republicans are not voting on constitutionality or precedent, they are voting on desperately needed Border Security & the Wall. Our Country is being invaded with Drugs, Human Traffickers, & Criminals of all shapes and sizes. That's what this vote is all about. STAY UNITED!" he said.