The Constitution is silent on the specific topic of abortion. It does, however, in the Fourteenth Amendment guarantee a right to due process under the law. This is intended to prevent the government from enacting any law that results in an arbitrary denial of life, liberty, or property by the government outside the sanction of law.
In Roe v Wade the decision of the court was (simplified) that any law against abortion represented a reach of the government into the womb.
With respect to gun rights, it's probably instructive to quote the text itself (as it's so short):
The intent of the law is pretty clear, however the gun lobby have pushed the letter of the law as far as possible.
He has as much power as any US President ever does. The US congress has basically been gridlocked since the late 1990s. Neither side has been able to obtain both a working majority in the House and a super-majority in the Senate at the same time, so any legislation that makes it as far as a vote inevitably dies in one house or the other. If Machin and Sinema could be counted on to actually vote the party line then there might be a chance - though that would require either the filibuster rule to be suspended/revoked or they have to use budget reconciliation rules. The problem with the latter is that it can only be done once per Congress, so the GOP would love nothing more than to force the Democrats to use 'early' as that would kill any chance whatsoever of getting anything done past that point.