The Right to Vote in the US

It would seem odd that a country which is so proud of its democratic process and wants to impose that on "dictatorial" nations. I want to remind you of the characteristics of both a republic and a democracy before you start on the "republic not a democracy" bullshit:

1. A political system for choosing and replacing the government through free and fair elections;
2. The active participation of the people, as citizens, in politics and civic life;
3. Protection of the human rights of all citizens, and
4. A rule of law, in which the laws and procedures apply equally to all citizens.

There is NO difference between the two in modern political thought: well short of republics not having aristocracy.
Anyway, there is no affirmative right to vote. There are amendments to the U.S. Constitution that prohibit discrimination based on race (15th), sex (19th) and age (26th), but that's kind of meaningless if everybody is excluded.
On the other hand, a right to vote is a cornerstone in international law, Significant international treaties, including the International Covenant on Civil and Political Rights and regional agreements such as the American Convention on Human Rights, enshrine citizens' claim to universal and equal suffrage. The most important of these documents is The Universal Declaration of Human Rights.
The Universal Declaration of Human Rights was adopted unanimously by the United Nations General Assembly in 1948. It recognises the integral role that transparent and open elections play in ensuring the fundamental right to participatory government. Article 21 states that:

Everyone has the right to take part in the government of his/her country, directly or through freely chosen representatives. Everyone has the right of equal access to public service in his country. The will of the people shall be the basis of the authority of government; this will shall be expressed in periodic and genuine elections which shall be by universal and equal suffrage and shall be held by secret ballot or by equivalent free voting procedures.

The problem here is that US law hasn't caught up with the rest of the world, despite the US having been signatory to some of these agreements, which means they would be constitutional law under Article VI.
Perhaps the situation would be different in the US if there was recognition for this right.