Warren, Sanders and Klobuchar are all grounded from campaigning during this trial. The other 3 are not.
It would not just be the witnesses Schiff wants, but any witnesses Trump wants - meaning all of the Bidens, Schiff, the whistleblower, Nadler, and since the House uses hearsay and speculation, but general attacks against Trump's dealing with foreign policy, in rebuttal Trump's team could call anyone they want to for hearsay, speculation and praise of Trump on Ukraine and his foreign policy.
The trial could last for months - all the way thru the Democratic Primary with the 3 Senators completely grounds and out of the news cycle.
Once Biden had it locked down, and the trial still going, the Senate Republicans could force Biden, his children and his brother all to testify about ALL the countries and ALL the domestic government projects that Biden spearheaded for the tax dollars - as at the same time his kids negotiated getting a cut of those tax dollars. PLUS, of course, Republicans could call hearsay, speculating and condemning witnesses against Biden - basically Benghazi-hearing Biden directly for days and indirectly including thru his kids for weeks.
Thus, the trial ends with Trump exonerated and the Republicans in the Senate having turned the trial into the question of "just HOW CORRUPTION is Joe Biden and how much money tax money has his relatives stolen thru the most well recognized form of graft? The Democratic Senator candidates are taken off the campaign and silenced, while the trial ends up being about Biden, not Trump.
It has been reported that some Democrats in the Senate have said if calling witnesses includes Biden, that won't vote for having witnesses - for obvious reasons. NOR could Democrat sob that the Republicans calling witnesses is unfair after they sobbed to have witnesses.
So, Democrats, are you certain you want a month's long trial controlled entirely by McConnell and the Republicans?