Democratic U.S. Sen. Elizabeth Warren and her Republican challenger, state Rep. Geoff Diehl went head to head in their final debate of the campaign.Warren, often mentioned as a possible 2020 presidential contender, is running for her second six-year term in office. Diehl co-chaired President Donald Trump's 2016 Massachusetts presidential campaign.Warren has been a frequent critic of Trump. Trump has also repeatedly criticized the Massachusetts Democrat in tweets and speeches.During one of the lighting rounds Tuesday, Warren was asked if she plans to serve her whole term if she is elected on Tuesday. She had previously said that she would “take a look” at running for the presidency after this election and repeated that Tuesday. “But I guarantee this, no matter what I do, I will work for the people of the commonwealth of Massachusetts.”“The answer for me is, ‘Yes,’” Diehl said. “But since she got a few extra words in there: Ironically, my opponent and Pres. Trump do have something in common. Neither one wants to be senator from Massachusetts, but both want to be president.”The remark earned a laugh from the studio audience and a smile from Warren.Warren tried to tie Diehl to the policies of President Donald Trump, who remains unpopular in Massachusetts. She said he failed to push back at key moments, including when Trump launched his presidential campaign by calling Mexicans rapists and murderers.Warren said Diehl "has Donald Trump's back" no matter how ugly the rhetoric.Diehl said Warren has turned her back on Massachusetts by saying she would take a hard look at running for president in 2020 after the November election. Diehl, who co-chaired Trump's 2016 Massachusetts presidential campaign, said his only goal is to represent the state in the Senate for the next six years.Diehl called Warren obsessed with Trump, saying "the fact of the matter is I am running for senate, not Donald Trump."Early in the debate, the candidates tackled a question about what gun laws, if any, should change in America in the light of mass shootings like the one at a Pittsburgh synagogue on Saturday. “The NRA will not permit us now in congress will not permit us to vote on things that most of America agrees on, background checks, deciding that terrorists should not be able to buy guns, bump stocks,” said Sen. Warren, who had the first opportunity to answer the question. “We’ve got things that we could do in Washington to help make all of our kids safer, but we’ve got to have the courage to stand up and do it.”In his first response, Diehl said laws existing in MA have done a good job keeping people safe but turned the topic to cracking down on illegal immigration and his endorsement from the Boston Police Association. He clarified the connection between those issues after being challenged by Warren. “A teenager was killed on an East Boston beach by an MS-13 gang member who was arrested about 3 weeks ago,” Diehl said. “Under Sen. Warren’s plan, there will be no agency that can actually identify and deport somebody whose committed that crime. When it comes to gun laws I do have a national priority, which is making national reciprocity happen.”Both were asked their reaction to Trump's announcement that he will send more than 5,000 military troops to the Mexican border to help defend against caravans of Central American migrants who are hundreds of miles away.Diehl said extra judges and other personnel should be sent to the border to help process the individuals to determine if they are legitimate refugees."One of the problems unfortunately with the refugee status is that it's been abused," Diehl said. "This caravan obviously is making its way to attempt to illegally cross in. Whether or not the entire group is refugees is hard to determine at this point."Warren said the country needs comprehensive immigration reform and blamed Trump for failing to take actions that could diminish the pressure felt by those hoping to gain refugee status."This has to do with Donald Trump's failed foreign policy overall. He has hollowed out the State Department and he has cut aid to Central America," Warren said. "That means the gangs are more powerful than ever. And so that means more people are fleeing the violence."The two were also asked about Trump's statement that he wants to sign an executive order to put an end to the constitutional right to citizenship for babies born in the United States to non-citizens."That's in the Constitution. The president can't end it by himself," Warren said. "He doesn't get to erase parts of the Constitution he doesn't like."Diehl said the county needs to follow the Constitution, which he said "does not call for providing citizenship to those who are not here as national citizens" and their children.Warren returned several times during the debate to the issue of the Republican-led tax cut approved in 2017.Warren said the plan mainly benefited the rich and corporations, and undercut other efforts, including a campaign promise by Trump to push through a plan to put people to work strengthening the nation's transportation networks."Where's all that money that was promised for infrastructure?" Warren said. "It's all gone to the billionaires and the giant corporations."Diehl pointed to his role in helping lower the tax burden on Massachusetts drivers by pushing a 2014 ballot question that successfully repealed a 2013 state law that would have allowed for automatic annual increases in the state's gas tax by indexing the tax to the rate of inflation.Diehl also noted that Massachusetts has been experiencing an economic boom with Trump in the White House.Election Day is Nov. 6 and early voting has begun in locations across the state.The Associated Press contributed to this report.