What is "Tanchem"?
Rashi: It leads them.
R. Yonah: They will not need to toil to let their purity guide them. The results of their straightness strengthen their purity!
Who are "Yesharim"?
R. Yonah: Yashar is one who loves straightness, and chooses Tzedek and what is correct. It includes recognizing straightness, and his heart is truly proper. To love and choose something, he must recognize it! Straightness includes two matters. (a) His nature leans to straightness in all its investigations, and (b) also his desires. Perfection of a man is to bring Chachmah and desire for straightness. It also must desire bodily needs. One must strengthen himself to draw his desires to straightness, and remove his leaning to other desires and pleasure. He should remove other desires, in thought and action, except for what is needed to sustain the body and the health of his limbs. This goes together with increasing desire for Chachmah, and constantly thinking about straightness. Then, desire for pleasure ceases. Yosher includes all Midos Tovos.
Malbim: Yashar is one whose heart goes in a straight path, (a) in Emunos, mindsets and matters of Binah. This is why Yosher is usually said regarding Binah, and (b) in Midos and deeds whose source is in Chachmah. This distinguishes Yashar from Tzadik. Yashar, his heart leans to good due to straightness planted in his heart. A Tzadik learned Tzedek and became used to it. He conquers his Yetzer and accustoms himself to act the opposite of his nature. A Yashar does not look at the laws of Tzedek, rather, the law of straightness; he goes beyond the letter of the law, like Chazal expounded 1 from "v'Asisa ha'Yashar veha'Tov" (Devarim 6:18).
Rashi (Devarim 6:18) says so, and Machzor Vitri cites so from Chazal. I did not find an explicit earlier source, but Bava Metzi'a 35a, 108a imply like this. (PF)
How does Tumas Yesharim lead them?
R. Yonah: Shlomo comes to teach about the Midos of Tam and Yosher. They are close to each other; many verses put them together - "Ish
R. Yonah: An animal without a blemish is called Temimah - "Tamim Yihyeh l'Ratzon" (Vayikra 22:21), "Parah Adumah Temimah" (Bamidbar 19:2). The perfection of an animal is not having a blemish. A man's blemish is bad Midos of the Nefesh. One who has no lowly Midah is called Tamim. It says about a Tzadik Tamim 'Tamim in his ways, and Tzadik in his deeds' (Avodah Zarah 6a).
What is "Yeshadem"?
Rashi: It plunders them.
R. Yonah: This is a curse, due to the great lowliness of their Midah.
R. Yonah (4): It will eradicate them. They will not attain a straight path until they uproot and remove Selef.
How does "v'Selef Bogedim Yeshadem"?
R. Yonah: Begidah (betrayal) is the opposite of straightness - he repays evil for good. Selef Bogedim is stubbornness and switching from good Midos [to bad] via the Midah of Begidah. Every day they strengthen in Begidah and their desire to reverse straightness. Via this, they acquire bad Midos; it causes that they will not return to good - "ve'Evilim b'Chasar Lev Yamusu" (10:21).
R. Yonah (4): Selef is the opposite of Yashar. It is the 'offspring' of Bogedim.
Malbim: Bogedim are the opposite of Yishrei Lev. They go the opposite of straightness. Selef is one who ruins straightness and Tzedek, like a bandit on a crooked, deserted path. He plunders the Nefesh's wealth, and casts it into the pit of destruction.
