The moderators of the next Democratic presidential debates, to be broadcast by CNN on July 30 and 31, should ask the candidates the following question: “If you were elected president next year, and the Democrats maintained control of the House and won a small majority in the Senate, would you support a change in Senate rules to eliminate the filibuster?”
The Senate now consists of 53 Republicans and 47 Democrats, the latter number including two independents, Angus King of Maine and Bernie Sanders of Vermont, who caucus with the Democrats.  To win control of the Senate, Democrats would need to pick up a net gain of three seats with a Democratic vice president, four seats otherwise.  
For Democrats to win three more seats and rely on a Democratic vice president to break ties, Democratic candidates would need to win three out of four difficult races: re-electing Doug Jones in Alabama (easier if Roy Moore ends up as the Republican nominee again), and defeating Republican incumbents Martha McSally in Arizona, Cory Gardner in Colorado, and Susan Collins in Maine.
Even if Democrats organize the Senate by the smallest of margins, Mitch McConnell will continue as Republican leader – his defeat in Kentucky is not likely. McConnell and the National Republican Senatorial Committee will do all they can to protect the GOP Senate majority, and will spend hundreds of millions of dollars to do so.  
But McConnell has also said that, if he should end up as minority leader rather than majority leader in the 117th Congress, his approach would be to shut down the Senate by filibustering every Democratic bill – similar to his blockade of President Obama’s nomination of Merrick Garland to the Supreme Court.
McConnell could easily hold together a group of at least 40 GOP Senators to make the obstruction-by-filibuster strategy work. (The one area where McConnell could not use the filibuster would be the confirmation of Supreme Court judges, because of recent rules changes that he himself instigated.)
There are some important actions that a newly-elected Democratic president could do without involving Congress at all – such as rejoining the Paris climate agreement, rolling back tariffs imposed by President Trump, and restoring some of the EPA regulations that were substantially weakened or eliminated during the Trump Administration.
However, many of the plans that Democratic candidates have proposed would require new legislation to be enacted – changes to the tax code, changes in the way health care is paid for, changes in the way higher education is financed, and changes in immigration policy, to mention just four important areas.  Even if such proposals passed a Democratic House, nearly all would be overcome by a Republican filibuster in the Senate. (Some changes might be possible through the budget reconciliation process, which requires only 51 votes, but many policies involve more than budgetary changes.)
Thus, the question about the future of the filibuster.  Allowing the Senate to pass legislation by 51 votes would certainly help a newly-elected Democratic president.  But it would substantially change the nature of the Senate. 
Would the short-term consequences of eliminating the filibuster be worth the longer-term risks of a future Democratic Senate minority not being able to use the filibuster?  Would there be other unintended consequences?  For example, if Democratic majority leader Harry Reid had not been able to get his colleagues to eliminate the filibuster for lower federal court judges late in the Obama years, would McConnell have been able to get elimination of the filibuster for Supreme Court justices through the Senate – and thus the confirmation of Neil Gorsuch and Brett Kavanagh by narrow majorities?  
But should the voters who supported a new president see that administration’s proposals come to naught because of opposition from a minority of senators?  Americans deserve to hear what Democratic presidential candidates have to say on questions such as these. 
Eric L. Davis is professor emeritus of political science at Middlebury College.