Just days after releasing their first draft, Senate Republicans issued a revised version of their bill to repeal and replace Obamacare.

GOP lawmakers issued a new iteration of the Better Care Reconciliation Act on Monday. Reuters and Politico reported the alterations were coming earlier in the day. The changes address ways to encourage people to buy insurance -- a key measure for keeping insurance markets stable and preventing premiums from skyrocketing.

The BCRA now includes six-month "lock out" period where people who don't have insurance have to wait a certain amount of time before their policy takes effect. The House bill would have allowed insurers to charge people who allow coverage to lapse up to 30% more for a year.

The Senate bill released last week scraps the individual mandate put in place by the Affordable Care Act that requires Americans buy insurance or pay a penalty. It keeps in place, however, the ACA's mandate that insurers must cover everyone.

As Vox's Sarah Kliff notes, the scenario could be a "death spiral" for the insurance markets, which need young and healthy people to sign up for insurance to keep premiums down for older and sicker people and protect themselves should they become sick.

Kliff also points out that the waiting period set to be included in the Senate bill could complicate the reconciliation process through which the GOP hopes to pass it. Reconciliation requires only a simple majority for passage (Republicans have a 52-48 majority in the Senate) but restricts which policies can be included to those that deal directly with government spending or revenue.

The revised bill also makes tweaks to stability funding for states.

Shares of insurers Aetna (AET) , Cigna (CI) - Get Report , Anthem (ANTM) - Get Report and Humana (HUM) - Get Report were all modestly lower in midday trading on Monday.

Majority Leader Mitch McConnell (R-KY) walks a tightrope in getting the healthcare bill through the Senate. The legislation has met fierce opposition among Democrats, and many Republicans have expressed doubts as well.

Republican Senators Dean Heller (R-NV), Ted Cruz (R-TX), Rand Paul (R-KY), Mike Lee (R-UT) and Ron Johnson (R-WI) have voiced opposition to the Senate bill. Senators Lisa Murkowski (R-AK) and Susan Collins (R-ME) have expressed concerns about the legislation as well, including over measures in it that would defund Planned Parenthood for a year.

The Congressional Budget Office is expected to release its BCRA score on Monday. The CBO estimated the number of uninsured people under the House healthcare bill would increase by 23 million by 2026 compared to under current law and reduce the deficit by $119 billion.

President Donald Trump tweeted about the healthcare bill on Monday, blaming its troubles on Democrats.