You introduce it now to setup being able to pass it later when you have a majority. The Republicans now have to officially go on record opposing it which means

You introduce it now to setup being able to pass it later when you have a majority. The Republicans now have to officially go on record opposing it which means you can use it as a campaign issue when running against them. If you then win a majority while having raised the issue during the campaign you can claim a "mandate" for passing it.