The GOP's Senate majority, which serves as President Trump's firewall against impeachment, once appeared reasonably secure in 2020. Strategists now warn it is increasingly vulnerable, in part due to fallout from the House impeachment inquiry.

As impeachment proceeds, vulnerable GOP senators will face a delicate balancing act: On the one hand, they must appeal to the independents and swing voters who may be open to impeachment. On the other hand, they must avoid alienating the avid pro-Trump voters who will not tolerate fainthearted support for the president.