WASHINGTON — Senate Republicans are seriously considering several last-minute changes to their tax legislation in an effort to mollify wavering members, four people familiar with the discussions said, as GOP leaders seek to keep their members from defecting ahead of crucial votes this week.

The lawmakers attracting the most concern from leadership and the White House are Senator Ron Johnson, Republican of Wisconsin, and Steve Daines, Republican of Montana, who say the current version of the bill favors corporations over other businesses.

There are numerous members demanding changes, and their needs don’t all overlap. Together, the requests put Republican leaders in a difficult position, as they attempt to accommodate individual holdouts on a one-off basis without losing other members or creating a situation in which the bill collapses under the weight of disparate demands.

Several factors complicated the leaders’ difficulty. On Sunday, an analysis released by the nonpartisan Congressional Budget Office showed the effects of the tax plan on lower-income Americans would be harsh.

That report mirrored one last week by the Joint Committee on Taxation. Both show poorer Americans eventually faring worse under the Senate plan; the CBO’s report also spotlighted how major changes to the Affordable Care Act — removing the health care mandate — would hurt these Americans.

Also challenging GOP leaders is the plan’s effect on the nation’s deficit. Its total size cannot be more than $1.5 trillion over a decade, so adding new benefits could force Republicans to find ways to raise additional revenue.

Presently, they have only about $80 billion in wiggle room to use, a small sum because many of the changes would be spread out over 10 years.

The four people spoke on the condition of anonymity because they were not authorized to discuss the internal discussions.

Johnson and Daines want changes to the bill they believe would help certain companies that file through the individual income tax code.

These companies, often known as ‘‘pass-throughs,’’ can be small businesses but also include larger firms with many employees. There are millions of such companies in the United States, and they account for the bulk of US firms.

Currently, in the Senate bill, these companies are allowed to deduct 17.4 percent of their income from their tax liability. Negotiators are looking at expanding that credit up to about 20 percent, two of the people said.

It’s unclear if Johnson would accept these changes as sufficient, and a final decision hasn’t been made. If Republican leaders believe Johnson’s demands have become too unreasonable, they could try and forge ahead without his vote. Spokesmen for Johnson and Daines didn’t have an immediate comment on the talks.

There are 52 Republicans in the 100-seat Senate, and GOP leaders can afford to lose only two votes if they want to pass their bill because Democrats are expected to unite in opposition against it.

Typically, tax bills require 60 votes in order to clear procedural hurdles, but Republicans are trying to pass the package through a process known as ‘‘reconciliation’’ that only requires a majority of votes.

At least six GOP members have raised concerns about specific provisions in the GOP tax bill, though they have not said they plan to vote against it this week. Johnson came closest, saying he opposed the measure but later suggesting he could support it with changes.

Senate Republicans have received criticism because their tax plan does not allow individuals, families, and pass-through companies to deduct their state and local taxes from their taxable income. The tax plan does allow firms that pay corporate income taxes to deduct their state and local taxes.

To create more parity, negotiators are considering putting new curbs on the ability of corporations to deduct state and local taxes from their income.

Voting on the tax measure is scheduled to begin Tuesday, with a procedural step in the Senate Budget Committee.