Without asking for any names...
Who this particular legislator is makes a big difference, because how you react, and the way you react, depends on that person's strengths and weaknesses.
Is this someone that is likely to hunker down and stonewall if they are pressured? Or are they someone who might rather get gunowner support in the next election. Might they be in a position to be concerned about a grass-roots effort to unseat them?
Generally speaking they aren’t so likely be concerned about the issue, and the right or wrong of it, but rather what their vote will mean in the way of political consequences later – both good and bad.
You might possibly point out what happened in Missouri. The Democrats controlled the Governor’s Mansion, but not the Legislature. When the Republican–controlled Legislature passed a CCW bill the Governor vetoed it, and made a big splash of it. But following this he lost his own party’s primary, and when the 2004 elections were over, a Republican was elected Governor, and his party’s majority increases in the Legislature.
The CCW issue worked against the Democrats, and left them with no way to advance their agenda, or block attacks on it. The main reason they lost was because they thought the Kansas City and St. Louis vote would carry the day for them, and underestimated the rural vote. There are obvious parallels here with Wisconsin, and I would expect that a Democrat might be particularly concerned about history repeating itself.
If this Democrat is in a safe district you might point out that while you may not be able to touch him or her, you can go after others. If the party’s control is diminished anywhere all of the survivors will be affected. On way or another, you can reach them, and you will.
Politics is all about power, not legislation. Someone’s got it and wants to keep it. Someone else is out to take it away for themselves. You can guarantee that if this bill goes under there will be a backlash. If you lose now, they will loose later. The trick is to convince (whoever) this is the way it will be. On the other hand if they stand with you, you won’t forget it.
I wish you luck.