The fight over immigration enforcement is moving to the Senate, where Democratic opposition will be tested.

The House passed a pair of immigration bills late last week: “Kate’s Law” to increase maximum penalties for criminal aliens who attempt to re-enter the country, and a second bill cutting funding to cities that refuse to comply with federal immigration laws.

Republicans got an unexpected boost when two-dozen House Democrats voted for “Kate’s Law,” viewed by GOP supporters as a first step toward implementing President Trump’s campaign promises on immigration.

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The defections came after House Democratic leaders said they wouldn’t twist arms to get their members to oppose the legislation. But the outcome is raising questions about whether Democratic senators up for reelection in 2018 will similarly break rank as the fight shifts to the upper chamber.

Senate Democrats are expressing confidence that they’ll be able to block the bills if they are brought up for a vote.

"I will do whatever I can in order to stop them. These are only punitive in nature, they don't deal with the totality of the reality of our immigration challenge, and as a continuing part of the Republican saga that only looks at one element, and looks at it in a way that is totally disproportionate," Sen. Bob Menendez Robert (Bob) MenendezKasie Hunt to host lead-in show for MSNBC's 'Morning Joe' Senators ask for removal of tariffs on EU food, wine, spirits: report VOA visa decision could hobble Venezuela coverage MORE (D-N.J.) said.

Democrats previously blocked similar proposals in 2015 and 2016. But a renewed push could force the 10 senators running for reelection in purple and red states won by Trump to take a tough, politically controversial vote.

A top House Democratic aide predicted that “Kate's Law” would be used in campaign ads against vulnerable Democrats.

"The ad writes itself," said the aide. "They're gonna use Kate Steinle's picture in a Willie Horton-style ad," referring to a controversial 1988 TV ad used by President George H.W. Bush's campaign against Michael Dukakis.

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Red-state Democrats are remaining tightlipped about the two immigration bills amid the fight in the Senate over repealing ObamaCare.

But Democratic Sens. Joe Donnelly Joseph (Joe) Simon DonnellyEx-Sen. Joe Donnelly endorses Biden Lobbying world 70 former senators propose bipartisan caucus for incumbents MORE (Ind.), Joe Manchin Joseph (Joe) ManchinBiden promises Democratic senators help in battleground states Senate leaders quash talk of rank-and-file COVID-19 deal OVERNIGHT ENERGY: House Democrats tee up vote on climate-focused energy bill next week | EPA reappoints controversial leader to air quality advisory committee | Coronavirus creates delay in Pentagon research for alternative to 'forever chemicals' MORE (W.Va.) and Heidi Heitkamp Mary (Heidi) Kathryn HeitkampCentrists, progressives rally around Harris pick for VP 70 former senators propose bipartisan caucus for incumbents Susan Collins set to play pivotal role in impeachment drama MORE (N.D.) previously voted to take up a bill toughening penalties on some undocumented immigrants who illegally re-enter the country after being deported.

A senior Senate Democratic aide questioned if Senate Majority Leader Mitch McConnell Addison (Mitch) Mitchell McConnellObama calls on Senate not to fill Ginsburg's vacancy until after election Planned Parenthood: 'The fate of our rights' depends on Ginsburg replacement Progressive group to spend M in ad campaign on Supreme Court vacancy MORE (R-Ky.) would bring up the House bills with Democrats still largely opposed to tougher immigration proposals.

“Nobody has changed their view on that to my knowledge. I'm not sure McConnell will even bring them up,” the aide said when asked if Democrats would be able to block the House bills similar to previous votes.

But Senate Republicans will likely face growing pressure to move a bill cracking down on illegal immigration after Trump praised the House-passed bill.

“Now that the House has acted, I urged the Senate to take up these bills, pass them, and send them to my desk. I am calling on all lawmakers to vote for these bills and to save American lives,” Trump said after the House’s vote.

Trump repeatedly invoked the shooting of 32-year old Steinle on the campaign trail to promote his immigration agenda, including during his speech at the Republican National Convention last summer when he accepted the party’s nomination.

Steinle was fatally shot in 2015 in San Francisco by a man who had had seven previous felony convictions and was deported to Mexico on five previous occasions.

The House bill could have competition for Senate floor time as lawmakers prepare to return for a jam packed three-week session where they have a slew of must-pass bills and a looming fight over Trump’s FBI nominee.

But conservatives are already clamoring for the Senate to try to crack down on illegal immigration. Sen. Ted Cruz Rafael (Ted) Edward CruzSenate Republicans face tough decision on replacing Ginsburg Cruz: Trump should nominate a Supreme Court justice next week Renewed focus on Trump's Supreme Court list after Ginsburg's death MORE (R-Texas) wants a vote this year and has introduced his own version of “Kate’s Law.”

“The House of Representatives took a tremendous step today to protect our national security and ensure the safety of our communities by passing Kate’s Law,” Cruz said after the House’s vote. “I look forward to the Senate swiftly taking up this bill and hopefully, passing it.”

Unlike the House bill, Cruz’s version includes a five-year minimum sentence for any undocumented immigrant who re-enters the country illegally after previously being deported twice or convicted of an aggravated felony.

A spokesman for McConnell said he didn’t “have any [scheduling] announcements yet this week.”

Sen. John Cornyn John CornynBipartisan praise pours in after Ginsburg's death Chamber of Commerce endorses McSally for reelection Airline job cuts loom in battleground states MORE (R-Texas), the No. 2 Senate Republican and a close ally of McConnell’s, is also working on a broader border security bill that is expected to touch on illegal immigration.

The bill, which is still being crafted, includes mandatory minimum sentences for certain immigrants who try to re-enter the country after being deported, according to a draft copy of the legislation.

A spokesperson for Cornyn confirmed on Monday that the bill would also target funding for cities that don’t comply with federal immigration policy but said “that provision is different than the bill passed by the House last week.”

Republicans will need to win over at least eight Democratic senators to pass any immigration or border security bill. Spokespeople for Manchin and Donnelly didn’t respond to a request for comment about their positions.

If the three Democratic senators did vote with Republicans on a proposal to impose tougher penalties on undocumented immigrants re-entering the country illegally, GOP leadership would still need to flip an additional five senators.

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Their targets would likely include Democratic Sens. Claire McCaskill Claire Conner McCaskillDemocratic-linked group runs ads in Kansas GOP Senate primary Trump mocked for low attendance at rally Missouri county issues travel advisory for Lake of the Ozarks after Memorial Day parties MORE (Mo.), Bob Casey Robert (Bob) Patrick CaseySecond GOP senator to quarantine after exposure to coronavirus GAO report finds brokers offered false info on coverage for pre-existing conditions Catholic group launches .7M campaign against Biden targeting swing-state voters MORE (Pa.), Sherrod Brown Sherrod Campbell BrownBipartisan praise pours in after Ginsburg's death Emboldened Democrats haggle over 2021 agenda Hillicon Valley: Russia 'amplifying' concerns around mail-in voting to undermine election | Facebook and Twitter take steps to limit Trump remarks on voting | Facebook to block political ads ahead of election MORE (Ohio), Bill Nelson Clarence (Bill) William NelsonDemocrats sound alarm on possible election chaos Trump, facing trouble in Florida, goes all in NASA names DC headquarters after agency's first Black female engineer Mary W. Jackson MORE (Fla.) and Jon Tester Jonathan (Jon) TesterPence seeks to boost Daines in critical Montana Senate race This World Suicide Prevention Day, let's recommit to protecting the lives of our veterans Filibuster fight looms if Democrats retake Senate MORE (Mont.) — each up for reelection next year in states won by Trump.

There are already early signs that a vote for or against the bill would be prime fodder for the 2018 election, where Democrats are defending 25 seats and Republicans are only protecting eight.

Sen. Dean Heller Dean Arthur HellerOn The Trail: Democrats plan to hammer Trump on Social Security, Medicare Lobbying World Democrats spend big to put Senate in play MORE (R-Nev.), up for reelection in a state won by Hillary Clinton Hillary Diane Rodham ClintonWhat Senate Republicans have said about election-year Supreme Court vacancies Bipartisan praise pours in after Ginsburg's death Trump carries on with rally, unaware of Ginsburg's death MORE, quickly hit two potential Democratic opponents for their votes against “Kate’s Law.”

“Congresswomen Dina Titus and Jacky Rosen disqualified themselves from public office. Let’s be clear, Congresswomen Titus and Rosen sided with violent criminals, some of which have committed horrendous crimes against Nevadans, over law enforcement and keeping our communities safe,” said Tommy Ferraro, a spokesman for Heller’s re-election campaign.

But Sen. Tim Kaine Timothy (Tim) Michael KaineBiden promises Democratic senators help in battleground states Second GOP senator to quarantine after exposure to coronavirus The Hill's Morning Report - Sponsored by The Air Line Pilots Association - Pence lauds Harris as 'experienced debater'; Trump, Biden diverge over debate prep MORE (D-Va.), who is also up for reelection, downplayed the chances that Democrats would help pass the bill as long as Republicans are unwilling to discuss broader immigration reforms.

“Democrats are not going to stand for harsh anti-immigration measures, we're not,” he said. “If they're not willing to meaningfully dialogue with us about immigration reform, you're not going to see us embracing their super-partisan anti-immigration bill.”