They do not have many supporters in the LGBT community, but it is certain that any supporters they do have will not support them now that they know that Trump and Rubio are supporting this summit. It is a shame that they are willing to go this far and that they have these beliefs.
Attending an event organized by any 'extremist' and 'anti-LGBT' organizations by Donald Trump and Ruby Rubio will definitely not augur well for the Republican Party this November.
The U.S government policy in particular as well as the attitude of the American public in general, based on followings of the happenings in the country in recent years, is tending towards supporting, or at least, advocating for full rights of the LGBT community.
Therefore, any anti-LGBT rally, and in this case 'extremist anti-LGBT summit' in America should not be attended by anyone running for the office of the President of the United States.
Trump and Rubio both plan to attend an "extremist anti-LGBT summit," which will not have a negative impact for them at the polls this November. Both appeal to the right wing, which does not support the LGBT, and will not vote for either one in November regardless of their attendance.
No, Trump and Rubio's plan to attend an "extremist anti-LGBT summit" will not likely have much negative impact for them at the polls this November for two reasons. First, many of their supporters are anti-LGBT and or extremists of one kind or another. Secondly, nothing Trump does ever seems to stick. He always has an excuse and others seem anxious to buy the excuses.