The democrats seek to temporarily replace her as she is currently recovering from shingles. The republicans refuse this temporary replacement but it will be put up for a vote. Why because it will hold the vote of Biden judicial nominees. That is the real reason why they refuse to support a temporary replacement.
They are trying to make an age argument. Yet Grassley is 89 and he has not committed to retiring. Other republican senators have expressed support for him to stay. He has decide to November.
So this is just another move made by the opposition to delay. Why would it matter if another democrat temporarily replace her until she returns to work? Its similar to them denying Obama from making a nomination to the Supreme Court because he lost the election but still the president and can make that decision.
When something similar happen with Trump who had lost the election and then the republicans pushed those judicial nomination thru at the last hour.